This invention relates generally to enhanced current densities by the formation of precipitates in ceramic-oxide superconductors to improve flux pinning and more particularly to the improvement in current densities by the enrichment of certain components in the compositions to promote the formation of precipitates in the Bi-Sr-Ca-Cu-O system.
High-T.sub.c superconductors in the Bi-Sr-Ca-Cu-O system have been reported to have low transport and magnetization critical current densities in polycrystalline form. The low transport property may be associated with the weak link effect resulting from the lattice misalignment and secondary phases at the grain boundaries, as in the case of YBa.sub.2 Ca.sub.3 O.sub.7-x. The magnetization critical current density has been found to be strongly dependent on field and temperature and is much lower than that of YBa.sub.2 Cu.sub.3 O.sub.7-x. This difference has been attributed to the lack of pinning centers, such as twin planes, in the Bi-Sr-Ca-Cu-O superconductors prepared by conventional ceramic techniques. Previous studies have also indicated that thermally assisted flux creep is much more pronounced in the Bi-Sr-Ca-Cu-O system. Significant resistance is present even at temperatures well below T.sub.c in a magnetic field due to the flux creep effects. The pinning centers in YBa.sub.2 Cu.sub.3 O.sub.7-x have been clearly identified as the twin boundaries. Therefore, it is of utmost importance to analyze the pinning mechanisms in this system so that additional pinning centers can then be introduced for the possible enhancement of the critical current density.
Critical current densities in ceramic superconducting compositions relate to both intergrain current density involving transfer between grains and to intracurrent density involving transfer within grains. Flux pinning is particularly important for increased intergrain current densities. Flux pinning sites may vary in different compositions.